How to remain the appended Brushes in grease pencil?
After appending, select the brush, open the Active Tool and Workspace Settings menu on the right and next to the brush there is this shield icon.
When you hover your mouse over it it displays:

Fake user. Save this data-block even if it has no users.

Make sure to click that so it's blue, and to do this to every different brush you want to use next time, or else Blender sees them as unused and doesn't bother saving them.